Conventional dental amalgams are among the most widely used restorative materials employed in dentistry. These amalgams are made by mixing a dental amalgam alloy with mercury, the amount of mercury varying from 40% to 60% by weight of the alloy, to obtain the necessary and desirable working quality or plasticity required, in accordance with the manufacturer's recommendations.
The standard or conventional amalgam alloy composition as currently certified by the American Dental Association (ADA) comprises:
Silver 65.0% minimum Tin 29.0% maximum Copper 6.0% maximum Zinc 2.0% maximum
An amalgam alloy of the above composition containing mercury up to 3.0% maximum in addition, is reportedly used extensively in European countries but is not very popular in the United States. Such alloys are described as "preamalgamated" alloys.
Metallurgically, the principal component of the standard or conventional amalgam alloy is the gamma phase, Ag.sub.3 Sn. The amalgamation reaction involves the solution of Ag.sub.3 Sn in mercury (Hg), from which a precipitation of silver-mercury (Ag.sub.2 Hg.sub.3, gamma-1 phase) and tin-mercury (Sn.sub.7 Hg, gamma-2 phase) takes place. The setting or hardening of the amalgam, which occurs in the tooth cavity, is associated with and responsive to these metallurgical changes.
The amount of tin-mercury (gamma-2) phase and the silver-mercury (gamma-1) phase increases with the amount of mercury added to the silver-tin (gamma) phase. In current clinical practice, wherein the mercury content ranges between 40-60% by weight of the amalgam alloy to which it is added, a major portion of the conventional alloy, in particulate form, will react with the mercury. Thus, in a hardened or set amalgam structure, one finds some unreacted silver-tin particles bonded or cemented together in a matrix of sliver-mercury and tin-mercury compounds. Trace amounts of a copper-tin complex have also been detected in the microstructure of a set amalgam.
The tin-mercury (gamma-2) phase has been held responsible for the tarnish and corrosion failure of the conventional dental amalgam. It has been recognized that this is due to an electrochemical polarization, resulting in the deterioration of the tin-mercury (gamma-2) phase and an eventual weakening of the physical structure. It has also been found that the tarnish of such dental amalgams results from the attack on the silver-mercury (gamma-1) phase by sulfide ions generated from ingested food materials.
It has further been observed and recognized that both the tin-mercury (gamma-2) and the silver-mercury (gamma-1) phases are relatively weak, and the brittle failure of amalgam restorations is believed to occur through the initiation of cracks in the tin-mercury phase and the inter-granular fracture of the silver-mercury phase. Moreover, it has been found that the silver-mercury phase is responsible for the creep of dental amalgams which in turn is related to marginal fracture. These defects of conventional dental amalgams account for the principal limitations of amalgam restorations.
A relatively new amalgam alloy is one disclosed in Youdelis U.S. Pat. No. 3,305,356 issued Feb. 21, 1967 for "Dental Amalgam". An alloy marketed under this patent has attracted widespread attention in view of the reportedly improved clinical behavior of the amalgams. One of these alloys as marketed consists of a mechanical mixture of two silver-rich alloys or components. The first of these is a conventional amalgam alloy of the silver-tin (copper-zinc) type. The second component is a silver-copper (AgCu) eutectic alloy, combined with the conventional alloy in a ratio of two parts of the conventional alloy to one part of the eutectic, by weight. The amalgam made by these components, when combined with mercury in the 40-60% proportions, is reported to contain very little or no tin-mercury (gamma-2) phase, but the amount of silver-mercury phase produced is not any less than that which is observed in a conventional amalgam for equivalent mercury content. Other microstructural constituents believed to be present in this amalgam include copper-tin complex, and unreacted silver-tin (gamma) phase and silver-copper (AgCu) eutectic particles.
According to the Youdelis patent disclosure and as expected, any significant oxidation of the silver-copper eutectic tends to retard amalgamation and unduly increase the setting time of the amalgam. Further, it has been reported that if the copper concentration in the conventional dental alloy exceeds a maximum of 6.0%, as stated in the ADA specification, the resultant amalgam exhibits excessive expansion and a greater tendency to tarnish. These undesirable qualities or characteristics, based upon a copper content of greater than 6.0%, appear to be due, at least metallurgically, to the presence of Ag.sub.5 Sn (beta phase) in the alloy microstructure. A correlation has been found to exist between the presence of such a beta phase and an uncontrollable (i.e., unrestricted) expansion of the amalgam. Further, the Ag.sub.3 Sn (gamma) phase has a sulfide tarnish resistance greater than that found in silver-tin alloys containing higher amounts of silver (Ag.sub.5 Sn-beta phase is such an alloy). Moreover, the basic silver-tin (gamma) phase has the highest physical strength properties of all the major components of the standard or conventional ADA dental amalgam, including the silver-mercury and tin-mercury, gamma-1 and gamma-2, phases.